In a Long Term Evolution (Long Term Evolution, LTE) system, a random access channel (Random Access Channel, RACH) is mainly used for initial access of a user equipment (User Equipment, UE) and does not carry any user data. A signal sent by a UE on an RACH is a preamble (Preamble) sequence, where the preamble sequence is a Zadoff-Chu sequence (Zadoff-Chu sequence, ZC sequence). In the prior art, a Preamble may include two parts which are a section of cyclic prefix (Cyclic Prefix, CP) with a length of TCP and a section of access sequence (Sequence, SEQ) with a length of TSEQ. In addition, parameter settings of different formats of Preambles may be matched to different cell radii, as shown in Table 1:
TABLE 1Preamble sequenceMaximumformat No.TCPTSEQcell radius (km)0 3168 · Ts24576 · TsApproximately 14.6121024 · Ts24576 · TsApproximately 77.32 6240 · Ts2 · 24576 · TsApproximately 29.6321024 · Ts2 · 24576 · TsApproximately 1004 448 · Ts 4096 · TsApproximately 1.4
where Ts is a basic time unit in an LTE protocol, and Ts=1/(15000×2048)s.
In the prior art, a 0-15 km/h low speed scenario is optimized by the LTE system, so that relatively high performance is still achieved in a 15-120 km/h high speed movement scenario, and connection can still be maintained in a 120-350 km/h high speed movement scenario. In an existing LTE protocol, two cell configurations, an unrestricted cell configuration and a restricted cell configuration, are supported, where an unrestricted cell is applied to a low frequency deviation scenario (for example, the frequency deviation is less than 600 Hz), and a restricted cell is applied to a high frequency deviation scenario (for example, the frequency deviation is greater than 600 Hz). With regard to a restricted cell, when a random access signal sent by a UE uses a ZC sequence (Zadoff-Chu Sequence) as a random access sequence, an evolved base station (evolved Node B, NodeB or eNB or e-NodeB) can ensure correct detection of a round trip delay (Round Trip Delay, RTD) within a frequency deviation range
      [                  -                              3            *            Δ            ⁢                                                  ⁢                          f              RA                                2                    ,                        3          *          Δ          ⁢                                          ⁢                      f            RA                          2              ]    ,where ΔfRA represents a subcarrier spacing of the random access channel, and the UE adjusts a timing advance (Timing Advance, TA) according to the RTD, thereby adjusting message sending timing and ensuring that the UE can normally access a network.
With the development of communications technologies and increased communications requirements of users, operators come up with requirements for coverage in very-high-speed movement scenarios and high frequency band high-speed railway scenarios. In the two types of scenarios, a frequency deviation of the random access signal is larger, which is
      [                  -                              W            *            Δ            ⁢                                                  ⁢                          f              RA                                2                    ,                        W          *          Δ          ⁢                                          ⁢                      f            RA                          2              ]    ,where W≧5. It is very difficult for an eNB to ensure correctness of RTD detection under a high frequency deviation. As a result, it is very difficult to ensure that a UE normally accesses a network, which affects access performance of the network.